Stabilizing high-order harmonic mode-locking in Er/Yb-doped fiber lasers with sub-MHz fundamental frequency through optoacoustic resonance

This study explores harmonic mode-locking (HML) in an Er/Yb-doped fiber laser with a sub-MHz fundamental frequency of 0.678 MHz. By tuning a polarization controller, we identified regimes where multiple soliton bunches achieved low-order HML (3rd to 8th harmonics) and a remarkable high-order HML regime at the 472nd harmonic, corresponding to a 320 MHz repetition rate. This regime demonstrated outstanding stability, with a supermode suppression level of 49 dB and timing jitter on the order of a few picoseconds. The exceptional stability is attributed to an exact optoacoustic resonance between the laser repetition rate and intrinsic fiber acoustic modes (R06 and TR2,15). These findings represent a significant advancement in the development of fiber lasers with high repetition rates and robust pulse trains, offering practical solutions for applications in spectroscopy, telecommunications, and microwave photonics.